Classic Tannoy DC's roll off quite sharply at about 15/16kHz.

Anyone who thinks Tannoy could produce a CD that was intrinsically flat from 1.2kHz to 20Hz is living in cloud cuckoo land.

The ageing Tannoy crossovers can be improved upon but its more about flattening the response than increasing the frequency range.

This link gives some good curves of what is achievable.

http://www.troelsgravesen.dk/tannoyMG15.htm

I've had 15" Tannoy DC's in large cabinets, used in both passive and active configuration, and IMHO they are better than most current day designs in many ways. However, I prefer BTH K10A's as they produce more articulate bass than the Tannoy's, are comparable in the mids and articulate HF much better than the Tannoy's, even though they only go about 1kHZ higher. All in all much more enjoyable speaker to listen to but unfortunately not as common as the Tannoy's.

Just my personal experience though.